As the title says. Well, Mage Revised doesn't even say they're double-cost - but Guide to the Technocracy says:
Like the Background: Device (Talisman), each dot in Enhancements costs two Background points, not one; the modifications are essentially built-in devices.
Device does not clarify, merely pointing to the Talisman background in the Mage corebook which, near as I can tell, does not actually exist - unless it's in the pre-Revised edition?
At any rate, assuming it is, in fact, double-cost, why is it double cost? What's the game balance reason?